People rely on disposable absorbent articles to make their lives easier. Disposable absorbent articles, such as adult incontinence articles and diapers, are generally manufactured by combining several components. These components typically include a liquid-permeable topsheet; a liquid-impermeable backsheet attached to the topsheet; and an absorbent core located between the topsheet and the backsheet. When the disposable article is worn, the liquid-permeable topsheet is positioned next to the body of the wearer. The topsheet allows passage of bodily fluids into the absorbent core. The liquid-impermeable backsheet helps prevent leakage of fluids held in the absorbent core. The absorbent core generally is designed to have desirable physical properties, e.g. a high absorbent capacity and high absorption rate, so that bodily fluids can be transported from the skin of the wearer into the disposable absorbent article.
Frequently one or more components of a disposable absorbent article are adhesively bonded together. For example, adhesives have been used to bond individual layers of the absorbent article, such as the topsheet (also known as, for example, the body-side liner) and backsheet (also known as, for example, the outer cover), together. Adhesives have also been used to bond discrete pieces, such as fasteners and leg elastics, to the article. In many cases, the bonding together of components forms a laminated structure in which adhesive is sandwiched between materials (such as layers of polymer film and/or layers of woven or nonwoven fabrics) that make up the components being bonded together.
In many instances, a hot-melt adhesive, i.e. a polymeric formulation that is heated to substantially liquefy the formulation prior to application to one or both materials when making a laminate, is used in making a laminated structure. While such formulations generally work, they can be costly and their performance properties can be improved. For example, adhesion can be improved to help provide a sturdier laminate (e.g., to improve the integrity or strength of the bond between two components in a disposable absorbent article).
There is a need or desire for an adhesive composition that possesses one or more performance characteristics that are comparable to, or better than, one or more of the same performance characteristics (e.g., bond strength) of a conventional hot-melt adhesive and that will typically cost less than a conventional hot-melt adhesive. Laminated structures and disposable absorbent articles employing the adhesive composition would benefit from these improved characteristics. There is also a need or desire for efficient methods of making the adhesive composition, and efficient methods of making laminated structures and disposable absorbent articles employing the adhesive composition.
The present invention is generally directed to amorphous polyalphaolefin adhesive compositions having improved bonding strength through the addition of crystalline polypropylene. The adhesive compositions have better performance characteristics, e.g. shear and peel bonding strengths, than conventional hot-melt adhesives, and may cost less than conventional hot-melt adhesives.
The combination of amorphous polyalphaolefin (APAO) and crystalline polypropylene possesses desirable adhesive properties and may be used to make laminated structures and disposable absorbent articles. The adhesive compositions of the invention can be applied to a wide variety of substrates, including nonwoven webs, woven webs, and films. The adhesive can be applied in a swirl pattern, can be melt-blown, or can be applied using any technique suitable for hot-melt adhesives.
Without being bound to any particular theory, it appears that such a great difference between bonding strength of the adhesive compositions of the invention and conventional amorphous polyalphaolefin adhesives may be attributed to crystallization of crystalline, or isotactic, polypropylene, which generates physical intermolecular linking in the matrix of APAO.
As stated above, a material comprising a combination of an APAO and crystalline polypropylene may cost less than a conventional hot-melt adhesive. Generally this is because conventional hot-melt adhesives are typically formulated by combining several components, including a polymer or polymers for cohesive strength; resins, tackifiers, or other generally low molecular-weight materials for adhesive strength; viscosity modifiers such as oils or wax-like materials; and other additives (e.g., antioxidants). In some versions of the invention, a combination of the APAO and crystalline polypropylene alone provides improved bond characteristics compared to conventional hot-melt adhesives. But it should be understood that the invention encompasses adhesive compositions that include selected amorphous polyalphaolefins and crystalline polypropylenes, combined with other additives or materials.
Another advantage present in some versions of the invention is that the material of the invention may be used in conventional hot-melt-adhesive processing equipment. Thus, the adhesive material may be used in equipment already installed for the purpose of processing and applying conventional hot-melt adhesives.
Apart from whether or not adhesive compositions of the present invention cost less than conventional hot-melt adhesives, we have found that representative embodiments of the present invention possess improved performance characteristics compared to the performance characteristics of conventional hot-melt adhesives. These performance benefits may justify processing and applying adhesive compositions of the present invention in modified conventional-hot-melt-adhesive equipment, or in equipment especially designed and built for the purpose of processing and applying adhesive compositions of the present invention. Furthermore, these performance benefits may justify adhesive compositions of the present invention, in some instances, being at a higher cost than conventional-hot-melt adhesives.